1. Field of the Invention
This application is related to the field of integrated circuit design, and more particularly to a software tool and method for analyzing the failure rate of an integrated circuit.
2. Description of the Related Art
Transistor devices in a circuit can undergo gate oxide degradation as a consequence of excessive voltage across the oxide during voltage overshoot and undershoot events. This degradation can cause excessive leakage in the device or even catastrophic damage of the gate oxide, causing circuit malfunction. This damage can manifest as a failure in the field of a product containing the circuit. Thus, as part of the product design flow, it is important to identify circuit devices which are susceptible to gate oxide breakdown caused by over-voltage during normal operating conditions. This is particularly important for products having circuits containing devices whose voltage rating is less than the voltage produced on I/O pins coupled to the circuits.
Various commercially-available simulation software tools can be used to simulate the operation of a circuit in order to identify the voltage and other operating conditions experienced by the circuit devices (e.g. transistors) during operation of the circuit. The operating conditions can then be used to compute a reliability rating or failure rate for each device. For example, the foundry which produces a device may provide a mathematical equation which takes into account variables such as the maximum voltage across the device, the time for which the maximum voltage was sustained, the temperature conditions in which the device is operating, the total time of operation, etc. Evaluating the equation computes a predicted failure rate for the device in the circuit based on these variables. Thus, the failure rate for an individual device in a circuit under given conditions can be determined fairly accurately (to the extent that the simulation and device equations are accurate).
This may help a circuit designer to determine the expected reliability of an individual circuit. However, the reliability of an individual circuit may not provide an accurate measure of how the circuit affects the reliability of a product in which the circuit is used. For example, many products use integrated circuits that can contain multiple instances of the same circuit. Thus, even if the predicted failure rate of each individual instance of the circuit is low, the possibility that one of the instances of the circuit can fail during normal operation may result in an unacceptably high failure rate for the overall product.